In the field of communications information is typically transmitted from a transmitter to a receiver by modulating a carrier waveform in dependence on the information. More particularly, modulation is the process of varying at least one parameter of a carrier waveform in order to convey the information. For example, it is known to vary the frequency, the amplitude or the phase of a carrier waveform to convey information. A wide variety of different modulation techniques are known in the art, each of which adopts a different convention of varying one or a number of parameters of the carrier in order to convey information. Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK) are two examples of known modulation techniques.
Digital modulation techniques, such as QAM and PSK, involve varying parameters of an analogue carrier signal according to the contents of a digital bit stream. In practice only a finite number of variations in the carrier signal are used by each different modulation technique to convey message information. Each different variation is known as a symbol and the collection of symbols forms the modulation scheme. When plotted on a phasor diagram the collection of symbol positions are referred to as the modulation “constellation”. In the case of PSK it is the phase of the analogue carrier signal that is varied in order to convey the contents of the digital bit stream and therefore, each different phase value used by each symbol is assigned a unique pattern of baseband bits. Usually, for a given modulation technique, each different symbol encodes an equal number of baseband bits. Thus a symbol corresponds to a particular sequence of baseband bits and typically represents a particular combination of phase and/or amplitude of the carrier signal. Each symbol has a particular location on a phasor plot of the carrier.
Improvements in digital telecommunications in recent years have resulted in the formation of modulation standards that each utilise a different modulation technique (or combination of techniques) to convey information between a transmitter and a receiver in a communications network. By using more than one modulation technique it is possible to improve communication between a transmitter and a receiver by, for example, increasing data transmission rates and improving data transmission reliability and accuracy. The E-GPRS standard teaches one such example wherein both differential GMSK and EDGE modulation techniques are used to modulate input data so that the data can be conveyed from a transmitter to a receiver and the data is accurately interpreted by the receiver. On the one hand, EDGE (also known as 3π/8-offset 8PSK) is linear and therefore, it is defined based on constellations. On the other hand, differential GMSK is non-linear so it is not defined based on constellations. However, it is known to approximate differential GMSK with π/2-offset 2PSK modulation which is linear and defined based on constellations.